The Oak Ridge National Laboratory waste heat generator relies on thousands of tiny piezoelectric cantilevers, similar to this one, produced at the École Polytechnique Fédérale de Lausanne (EFPL). (Source: EFPL)

Tiny cantilever generators could recycle as much as 30 percent of the heat chips put off

The Oak Ridge National Laboratory located about 23 miles south west of Knoxville, Tennessee has a storied history as the home of the Manhattan Project and birthplace of the atomic bomb. Today it has its sights set on many more tiny objectives, only this time it hopes to provide the world with cleaner energy, rather than destructive power.

The scientists at ORNL estimate that billions of dollars is lost every year in waste heat from electronic devices like central processing units (CPUs). Server farms spend millions a year to simply cool down their fields of racked computers. Waste heat is also a major source of energy loss in automobiles.

To that end, a team led by Scott Hunter has developed [press release] tiny piezoelectric generators that use a tiny cantilever, approximately 1 mm^2 in size. Approximately 1,000 of these tiny generators can be affixed to a 1 inch^2 CPU. Each tiny generator pumps out 1 to 10 milliwatts -- and together they can produce enough electricity to partially power fans or system sensors.

While that might not sound like much, it's actually significant progress.

Scientists have long hoped to use piezoeletric generators to recycle waste heat, but past designs have struggled with efficiencies, only able to convert between 1 and 5 percent of the waste heat to electricity. By contrast Professor Hunter's design has an efficiency of between 10 to 30 percent. The exact efficiency is dependent on the temperature of the waste heat generator (computer chip).

The tiny generator works using widely known physics principles. The tiny cantilever, anchored to the piezoelectric substrate bends due to the bi-material effect when heated. This is the same effect that traditional room thermostats rely upon.

As the cantilever bends it touches the heat sink, cooling it. It then flops back down on the hot surface. This flopping creates vibrations in the piezoelectric material that creates an alternating flow of current, which can be converted to usable DC current.

Describes Professor Hunter, "The tip of the hot cantilever comes into contact with a cold surface, the heat sink, where it rapidly loses its heat, causing the cantilever to move back and make contact with the hot surface. The cantilever then cools and cycles back to the cold heat sink. The cantilever continues to oscillate between the heat source and heat sink as long as the temperature difference is maintained between the hot and cold surfaces."

The key to higher efficiencies was to pick the right material and the right physical design. Professor Hunter comments, "The fast rate of exchange in the temperature across the pyroelectric material is the key to the energy conversion efficiency and high electrical power generation."

This kind of piezoelectric device is part of a growing class of devices known as micro-electrical-mechanical systems (MEMS). Professor Hunter envisions his new MEMS device being installed inside PC and server coolers, saving businesses and home users money on their power bills.

By improving the efficiency of electricity usage, the devices would also reduce fossil fuel usage. Comments Professor Hunter, "In the United States, more than 50 percent of the energy generated annually from all sources is lost as waste heat, so this actually presents us with a great opportunity to save industry money through increased process efficiencies and reduced fuel costs while reducing greenhouse gas emissions."

As the device is still pretty expensive, presumably, Professor Hunter is eyeing high-performance systems as the first target for commercial deployment. The tiny generator would allow additional cooling not currently available with a traditional block water-cooled system. By offering greater cooling to the chip, high performance mainframes could be run at higher speeds than ever before, a perfect proving ground for the new MEMS device.